The invention relates to processing video signals, more particularly, to detecting a source format of an incoming video signal.
It is often desired to display a movie originally recorded on film by means of a television broadcast. In order to comply with television broadcast field rates, some sort of film-to-video conversion must be performed. Typically, a movie is recorded and displayed at a frame rate of 24 frames per second. However, television broadcasts use a different rate, such as the 59.94 fields per second rate (i.e., 30 interlaced frames per second) of the NTSC standard where every two fields are interlaced and comprise one frame.
One method of converting film frame rates to television field rates is referred to as the “3:2 pull down” scanning method. A first film frame is scanned twice to make two fields, and then a second film frame is scanned three times to make three fields, the next frame twice, etc. To accommodate the fact that the NTSC vertical scan period is slightly less than 60 fields per second, the actual display rate may be slowed slightly.
FIG. 1 illustrates a segment of movie film being scanned for broadcast as an NTSC television signal. As indicated, the film displays 24 frames per second. Frame 1 has been scanned three times to make three fields of the television signal. Frame 2 has been scanned two times, Frame 3 three times, etc. The result is a television signal having 60 fields per second, which approximates the 59.94 field per second rate of the standard NTSC format. This process is referred to as “3:2 pull down scanning”.
Although the above description is in terms of 3:2 pull down scanning to an NTSC television signal, the same concepts apply to scanning movie films to other television formats. For example, for a PAL broadcast of 50 fields per second, a film-to-video ratio of 2 television fields per movie frame might be used. For this reason, the 3:2 pull down scan format is generally referred to as a “film-to-video format”, characterized by the fact that source image frames are scanned in a periodic sequence that results in a desired frame-to-field ratio. In the above example, the desired ratio is: (60/24)=(5/2). For integer frame numbers, this is equivalent to five fields for every two frames, with the best symmetry being accomplished with 3:2 pull down scanning.
At the receiving end of the television broadcast, when the television receiver includes digital processing components, a recent development has been the conversion of the incoming television signal into digital data for further processing. For example, some sort of motion compensation processing with video compression can be performed to prevent the viewer from perceiving visual artifacts as a result of motion in the scene being displayed. Additionally, film to video conversion with motion compensation is now being combined with techniques for digitizing the video data. These techniques include digital compression techniques requiring less bandwidth (bits per second) during transmission. Motion in the scene that was originally filmed is indicated if there is a change between adjacent fields. Those fields that represent the same movie frame will have no motion. However, at every boundary where a different movie frame has been scanned, the scene may change and there can be motion. Although various processing techniques have been developed for compensating the effects of motion between television fields, these methods are not generally designed for film-to-video formats.
One compression technique is embodied in a standard known as “MPEG”, named for the Moving Picture Experts Group that began the effort to provide a standard. The MPEG standard attempts to strike a balance between the high compression associated with interframe coding and the random access capability associated with intraframe coding. To answer this challenge, the MPEG standard uses two interframe coding techniques, predictive and interpolative, and an intra coded technique. For coding video data, the MPEG encoding techniques are used to encode MPEG “pictures” from fields or frames of video data.
However, the best processing choices for the particular video digitizing method depend on the format of the digitized television signal. In other words, the same processing algorithm might not be the best algorithm for data that represents film format as would be used for standard NTSC data. It is therefore necessary to detect when an incoming signal has a film format so that appropriate motion compensation processing can be performed.
In other application, de-interlacing is a video post-processing technique to provide progressive video output for progressive TV, such as LCD TV or Plasma TV. It is also necessary to detect when an incoming signal has a film format so that an appropriate interpolation method can be performed for the de-interlacing process. This detection and interpolation process is called inverse Telecine. Each frame of the progressive video output is generated by combining two fields originally from the same film frame. The output rate of the inverse Telecine processing is 60 frames per second for NTSC signal or 50 frames per second for PAL signal.